Heat treating device

ABSTRACT

A antireflective film  50  is formed on a thermocouple  42  arranged in a processing vessel  1  of a heat treatment apparatus in order to improve the transient response characteristics of the thermocouple  42 . In a typical embodiment, the thermocouple  42  is made by connecting a platinum wire  43 A and a platinum-rhodium alloy wire  43 B, and the antireflective film  50  is composed by stacking a silicon nitride layer  50 C, silicon layer  50 B and a silicon nitride layer  50 A in that order.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heat treatment apparatus, morespecifically to a technique that improves accuracy of temperaturemeasurement of the object to be processed, such as a semiconductorwafer.

2. Description of the Related Art

The semiconductor device manufacturing process employs several sorts ofheat treatment apparatuses that perform heat treatments, such asoxidation, diffusion, and film-forming. A batch-type, vertical-type heattreatment apparatus, which performs a heat treatment to a plurality ofobjects at one time, is known as one of the aforementioned heattreatment apparatuses.

With the batch-type, vertical-type heat treatment apparatus, a pluralityof semiconductor wafers are held by a wafer boat vertically spaced atintervals, and then the wafer boat is loaded into the processing vessel.The semiconductor wafers are heated up to a predetermined temperature bymeans of a tubular heater surrounding the processing vessel, and thenthe semiconductor wafers are subjected to a predetermined treatment. Atemperature controller controls the output power of the heater based onthe temperature data measured by a temperature sensor unit arranged inthe processing vessel, thereby the interior of the processing vessel,and thus the semiconductor wafers, is maintained at a predeterminedtemperature.

In order to obtain films having excellent property and uniform quality,uniform temperature distribution on each semiconductor wafer and betweensemiconductor wafers respectively placed at different heights isnecessary. Thus, the interior of the processing vessel is divided into aplurality of heating zones, and the output power of each heatercorresponding to each heating zone is independently controlled.

The temperature sensor unit used in such heat treatment apparatus iscomposed of: a straight protective tube made of quartz and extendingvertically in the processing vessel; and thermocouples arranged in theprotective tube at positions respectively corresponding to the heatingzones. The thermocouples measure temperatures at positions in theprocessing vessel respectively corresponding to the heating zones, andthe calorific power of the heaters is controlled based on the measuredtemperatures.

FIG. 6 shows a typical structure of a conventional temperature sensorunit. A thermocouple 60 is made by connecting a metallic wire 61A ofplatinum and a metallic wire 61B of platinum-rhodium alloy. The metallicwires 61A and 61B are inserted into tubular, electrically insulativemembers 62A and 62B, which are made of alumina ceramics. The metallicwires 61A and 61B extend within a protective tube 65 made of quartz, aredrawn outside the end part of the protective tube 65, and are connectedto a controller.

However, the transient response of the above temperature sensor unit isslow in view of the temperature rising rate of the semiconductor wafer,when they are being heated rapidly. If the rising of the temperaturemeasured by thermocouple is slower than the rising of the actualtemperature of the semiconductor wafer when the semiconductor wafer isheated, accurate temperature control is difficult, and thus a long timeperiod is required for the semiconductor wafers to reach a stablecondition in which they are at the predetermined process temperature.This lengthens the total processing time, resulting in reduction inthroughput.

SUMMARY OF THE INVENTION

The object of the present invention is to improve the transient responseof the temperature sensor of a heat treatment apparatus, thereby toprovide a heat treatment apparatus capable of adjusting the temperatureof the object quickly.

The inventors of the present investigated the cause of the slowtransient response of the temperature sensor. They found that the slowtransient response is caused by the fact that the reflectivity of themetallic wires (platinum wire and rhodium wire) of the thermocouple forthe radiant light emitted from the object to be processed is high andthus the emissivity of the metallic wires is low, due to the metallicluster of the metallic wires. Further, the inventors found that a heatcapacity of the thermocouple unit is increased due to provision of thetubular, electrically insulative member, which also results in the slowtransient response. The inventors concluded that the response of thethermocouple to the radiant light emitted from the object can beimproved by improving the emissivity of the thermocouple, and theycompleted the present invention.

The present invention provides a heat treatment apparatus including: aprocessing vessel; a heater adapted to heat an object to be processedplaced in the processing vessel; a temperature sensor unit arranged inthe processing vessel; and a controller adapted to control an outputpower of the heater based on a temperature data measured by thetemperature sensor, wherein the temperature sensor unit includes aprotective tube and a thermocouple having a pair of metallic wirescontained in the protective tube and connected to each other at aconnection, and wherein a film having an antireflective function isformed at least on a surface of the connection of the metallic wires.

The reflectivity of a portion, on which is the film is formed, of thethermocouple for a light of any wavelength within the range of 0.5 to5μm is preferably 80% or below, more preferably 50% or below.

Preferably, the film having the antireflective function is electricallyinsulative.

Preferably, the film having the antireflective function is composed of aplurality of layers. In this case, with respect to the thickness of thefilm, the uppermost layer is preferably electrically insulative, and alayer directly contacting the metallic wires is preferably made of amaterial inert to the metallic wire.

In a typical embodiment of the present invention, the metallic wires ofthe thermocouple may be made of platinum and platinum-rhodium alloy,respectively, and the film having the antireflective function may becomposed of a silicon nitride layer, a silicon layer and a siliconnitride layer formed on the metallic wires in that order. The siliconlayer may be doped with phosphate.

According to the present invention, the reflectivity of the temperaturemeasuring portion of the thermocouple can be reduced and the emissivitythereof can be increased.

In the event that the film includes an electrically insulative layer, itis not necessary to coat the metallic wires of each thermocouple withelectrically insulative tube, resulting in reduction in heat capacity ofthe thermocouple unit.

According to the present invention, the transient response of thethermocouple is improved, and thus the temperature of the object can bemeasured with high accuracy. Thus the heater can be controlledaccurately.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a vertical-type heat treatmentapparatus in one embodiment of the present invention, schematicallyshowing the structure thereof;

FIG. 2 is a cross-sectional view of a temperature sensor unit shown inFIG. 1, showing the structure thereof;

FIG. 3 is a cross-sectional view of a thermocouple of the temperaturesensor unit shown in FIG. 2, showing the structure thereof;

FIG. 4 is a cross-sectional view of the metallic wires of thethermocouple and a film formed on the connection of the metallic wires,schematically showing the structure thereof;

FIG. 5 is a graph showing a transient response characteristic of thethermocouple during heating operation; and

FIG. 6 is a cross-sectional view of a conventional thermocouple, showingthe structure thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A vertical-type heat treatment apparatus, which is one embodiment of thepresent invention and performs a film-forming process on semiconductorwafers by a CVD process, is described below with reference to thedrawings. FIG. 1 is a cross-sectional view of a vertical-type heattreatment apparatus according to the present invention, schematicallyshowing the structure thereof.

The vertical-type heat treatment apparatus comprises a processing vessel(processing tube) 11 of a dual-tube structure, which includes a straightinner tube 11A extending vertically and an outer tube 11B concentricallysurrounding the inner tube 11A with a certain gap therebetween. An upperend of the inner tube 11A is opened, and an upper end of the outer tube11B is closed. A space below the processing vessel 11 serves as aloading area L where semiconductor wafers are loaded onto and unloadedfrom a wafer boat 17, i.e., an object holder. The inner tube 11A and theouter tube 11B are made of a material that has excellent heat resistanceand corrosion resistance, such as quartz glass of a high degree ofpurity.

The processing vessel 11 includes a short cylindrical manifold 12 whichis connected to a lower end portion of the outer tube 11B. The manifold12 has a flange 12A at its upper end. A flange 111 is disposed at alower end portion of the outer tube 11B. The flanges 12A and 111 areconnected to each other by a flange presser 13 with a sealing means (notshown) such as O-ring being interposed between the flanges 12A and 111.

The inner tube 11A extends downward beyond a lower end of the outer tube11B to be inserted into the manifold 12, and is supported by an annular,inner tube support 14 disposed on an inner surface of the manifold 12.

A gas supply pipe 15 for introducing a process gas and an inert gas intothe processing chamber 11 passes through one side of a sidewall of themanifold 12 to extend upward in the inner tube 11A. A space between thesidewall of the manifold 12 and the gas supply pipe 15 is sealed in anairtight fashion. The gas supply pipe 15 is connected to a gas supplysource, not shown.

An exhaust port 16 for discharging air in the processing vessel 11 isdisposed on the other side of the sidewall of the manifold 12. Theexhaust port 16 is connected to an exhaust mechanism (not shown) whichhas, for example, a vacuum pump and a pressure control mechanism, bywhich a pressure in the processing vessel 11 is controlled.

Provided below the processing vessel 11 is an elevating mechanism 21,which vertically moves the wafer boat 17 to be loaded into and unloadedfrom the processing vessel 11. The elevating mechanism 21 includes adisk-shaped lid 20 adapted to close a lower end opening 11C of theprocessing vessel 11. The wafer boat 17 is made of quartz glass of ahigh degree of purity. Plural pieces, for example, about 100 to 150pieces, of semiconductor wafers are held on the wafer boat 17 whilevertically spaced at predetermined intervals, for example, 5.2 to 20.8mm.

A columnar support member 22 is attached to the lid 20 to extend upwardin parallel with an axis of the processing vessel 11. The support member22 passes through the lid 20. A disk-shaped boat support 22A adapted tosupport the wafer boat 17 thereon is provided integrally with thesupport member 22 at the upper end thereof. A lower end of the supportmember 22 is connected to a rotary driving means 23 provided below thelid 20. A heat insulating tube 24 is arranged on the lid 20, with thesupport member 22 passing through the heat insulating tube 24.

A tubular heater 30 is arranged outside the processing vessel 11 to heatsemiconductor wafers received in the processing vessel 11 up to apredetermined process temperature. The tubular heater 30 has acylindrical heat insulating member, not shown. Wire-shaped resistanceheaters are arranged on the inner surface of the heat insulating memberin a helical or serpentine pattern. The resistance heaters are connectedto a controller 31 which controls an electrical power to be supplied tothe wires in order to achieve a predetermined temperature for thesemiconductor wafers based on a temperature data measured by atemperature sensor unit 40, which is described below.

An interior of the processing vessel 11 is vertically divided into aplurality of heating zones. Four heating zones Z1 to Z4 are illustrated.The tubular heater 30 is capable of performing a so-called zone control.That is, the tubular heater 30 is capable of independently controllingtemperatures in the respective heating zones.

Each of the resistance heaters is arranged exclusively in one of theheating zones, and the controller 31 can independently controlelectrical power to be supplied to the resistance heaters.

A plane heater 32 is arranged above the processing vessel 11 in parallelwith an upper end surface of the tubular heater 30 to face the waferboat 17 in the processing vessel 11. The plane heater 32 prevents heatfrom being released from an upper part of the processing vessel 11, andthus the semiconductor wafers can be heated with an enhanced in-planeuniformity. The plane heater 32 has wire-shaped resistance heatersarranged on a plate-shaped base member. The resistance heaters are alsoconnected to the controller 31.

The processing vessel 11 has the temperature sensor unit 40 therein.Calorific powers of the tubular heater 30 and the plane heater 32 arecontrolled based on a temperature data measured by the temperaturesensor unit 40.

The temperature sensor unit 40 passes through the sidewall of themanifold 12 below the exhaust port 16. The sidewall of the manifold 12and the temperature sensor unit 40 are sealed in an airtight fashion.The temperature sensor unit 40 extends upward in an annular space formedbetween the wafer boat 17 and the inner tube 11A in parallel with aninner wall of the inner tube 11A. A tip portion of the temperaturesensor unit 40 is curved above an upper end of the inner tube 11A toextend toward a center of the processing vessel 11 in parallel withsurfaces of the semiconductor wafers held in the wafer boat 17.

As shown in FIG. 2, the temperature sensor unit 40 includes a protectivetube 41 made of transparent quartz glass, and a plurality ofthermocouples 42 (five thermocouples are shown) contained in theprotective tube 41. The thermocouples 42 are located at a positioncorresponding to a heating zone heated by the plane heater 32 (forexample, a position directly below a center of the plane heater 32), andpositions corresponding to the respective heating zones Z1 to Z4 heatedby the tubular heater 30.

The protective tube 41 has first portion 41A extending straight andvertically, a second portion 41B successive to an upper end of the firstportion 41A to extend in a horizontal direction (right direction in FIG.2) perpendicular to an axis of the first portion 41A, and a base endportion 41C successive to a lower end of the first portion 41A to extendhorizontally (opposite direction of the second portion 41B).

A distal end of the protective tube 41, i.e., a distal end of the secondportion 41B, is closed. A proximal end of the protective tube 41, i.e.,a proximal end of the base end portion 41C, is sealed by a sealant 45such as an adhesive. Metallic wires 43 of the respective thermocouples42 are drawn outside the protective tube 41 through the sealant 45. Themetallic wires 43 of the respective thermocouples 42 are connected toinput terminals of the controller 31 via compensating lead wires.

The base end portion 41C of the protective tube 41 may be sealed by thesealant 45 in an airtight fashion. In this case, the protective tube 41may be filled with inert gas, such as nitrogen gas (N₂ gas), forpreventing an oxidation of the thermocouples 42.

Alternately, an atmosphere of a reduced pressure may be established inthe interior of the protective tube 41. Thus, even if the protectivetube 41 is broken when the interior of the processing vessel 11 is at areduced pressure, the broken pieces of the protective tube 41 areprevented from scattering in the processing vessel 11.

Preferably, an annular groove (not shown) may be formed along the wholecircumference of the base end portion of the protective tube 41, and thesidewall of the manifold 12 may be fitted in the annular groove. Thus,when the processing vessel 11 is at a reduced pressure, the temperaturesensor unit 40 is prevented from being drawn into the processing vessel11.

Diameters of the metallic wires 43 of the thermocouples 42 are about 0.5mm, for example. Pairs of metallic wires 43 of each thermocouple arecomposed of a platinum (Pt) wire 43A and a platinum-rhodium alloy(Pt/Rh) wire 43B. As shown in FIGS. 3 and 4, a film 50 is formed onsurfaces of the respective metallic wires 43A and 43B, and on a surfaceof a connection 44 where the metallic wires 43A and 43B are connected toeach other, that is, a portion which measures a temperature.

The film 50 has an antireflective function for a light of any wavelengthwithin a predetermined wavelength range. When forming the film 50 havingthe antireflective function on the surfaces of the respective metallicwires 43A and 43B and the connection 44 thereof, a reflectivity of suchportion for a light of any wavelength within the range of 0.5 to 5 μm(micrometers) is low, as compared to the reflectivity of the surfaces ofthe respective wires 43A and 43B and the connection 44 thereof where thefilm 50 is not formed.

The film 50 may be composed of a plurality of layers of inorganicmaterials stacked with respect to the thickness of the film. Anuppermost layer 50A of the film 50 is preferably electricallyinsulative, and a lowermost layer 50C directly contacting the metallicwires 43A and 43B and the connection 44 thereof is preferably made ofsuch material that is inert thereto. The term “inert” herein means thatthe lowermost layer 50C does not substantially react with the metallicwires 43A and 43B and the connection 44 thereof at least within a rangeof a process temperature of the semiconductor wafers.

The film 50 having a desired function can be obtained by suitablyselecting substances of different refractive indexes and stacking thesame, considering the materials, the number, and the thicknesses of therespective layers 50A to 50C of the film 50.

The reflectivity of a portion of the thermocouples 42, on which the film50 is formed, for a light of any wavelength. within the range of 0.5 to5 μm is preferably 80% or below, more preferably 50% or below. In thisway, a response of each thermocouple 42 can be significantly improved.The reflectivity above 80% is not preferable, in that the rising of thetemperature of the thermocouples is slower than that of thesemiconductor wafers. It should be noted that light of the wavelengthwithin the range of 0.5 to 5 μm can heat the semiconductor wafers mostefficiently in a temperature range (100 to 1200° C.) for processingsemiconductor wafers.

More preferably, the film 50 is formed of the same, or substantially thesame material as that of the object to be processed, that is, thesemiconductor wafer. In this way, various characteristics of the film 50are close to those of the semiconductor wafer. Thus, temperature-raisingcharacteristics of the thermocouple 42 become close to those of thesemiconductor wafer.

From this point of view, the film 50 is preferably composed byalternately stacking, for example, a silicon nitride layer and a siliconlayer along the thickness direction of the film 50. In the embodimentshown in FIG. 4, the film 50 is composed by stacking three layers, i.e.,a silicon nitride layer 50A, a silicon layer 50B, and a silicon nitridelayer 50C. The silicon nitride layers 50A and 50C are 0.1 to 0.3 μmthick, the silicon layer 50B is 1 to 3 μm thick, and the overallthickness of the film 50 is in a range of 1.2 to 3.6 μm. That is, thethickness of the silicon layer 50B, which is formed of the same materialas that of the semiconductor wafer, is sufficiently thicker than thoseof the silicon nitride layers 50A and 50C in order that the reflectivityof the whole film becomes close to that of the semiconductor wafer. Thesilicon nitride, which forms the layers 50A and 50C, is selected becauseit is electrically insulative and inert to the metallic wires, asdescribed above. Since the reflectivity of the silicon nitride differsfrom that of the semiconductor wafer, the thicknesses of the siliconnitride layers 50A and 50C are made requisite minimum ones. It is notedthat the layers 50A and 50C are not made of completely differentmaterial from the silicon layer 50B, but made of silicon nitrideincluding silicon. The silicon layer 50B is preferably made of silicondoped with phosphate (P), for example. In this case, compared to thefilm 50 composed of a silicon layer which is not doped with phosphate,it is possible to reduce the reflectivity approximately by 20% for alight of any wavelength within the range of 0.5 to 5 μm, so that anexcellent antireflective effect can be obtained.

The number of the layers of the film 50 may also be 2 or less, or 4 ormore. The thicknesses of the respective layers are not limited to theabove embodiment.

The film 50 may also be formed of a material that has an excellentthermal conductivity, e.g., silicon carbide (SiC), aluminum nitride(AlN), and so on. A black-colored material is specifically preferable soas to effectively reduce the reflectivity.

A heat treatment for the object to be processed (semiconductor wafer)executed in a vertical-type heat treatment apparatus is described below.Loading of semiconductor wafers is carried out in the loading area L.The wafer boat 17 carrying the semiconductor wafers thereon is mountedon the boat support 22A when the lid 20 is in a lowermost position.Then, the lid 20 is elevated by the elevating mechanism 21, so that thewafer boat 17 is loaded into the processing vessel 11 from the lower endopening 11C, which is then air-tightly closed by the lid 20. Dummywafers are held on the uppermost and lowermost stages of the wafer boat17.

The exhaust mechanism reduces a pressure in the processing vessel 11 toa predetermined pressure, e.g., about 13Pa. The tubular heater 30 andthe plane heater 32 heat the processing vessel 11 to a predeterminedtemperature. When the rotary driving means 23 rotates the wafer boat 17,processing gas is introduced in the processing vessel 11 through the gassupply pipe 15. Then, the semiconductor wafers are subjected to afilm-forming process.

According to this embodiment, the film 50 is formed on the thermocouple43, that is, on the surfaces of the metallic wires 43A and 43B and theconnection 44 thereof. Thus, the reflectivity of the temperaturemeasuring part (connection 44) for the radiant light emitted from thesemiconductor wafers is reduced (the emissivity thereof is increased).

The temperature of each thermocouple is rapidly raised by receiving theradiant light emitted from the semiconductor wafers. That is, eachthermocouple is capable of measuring transient change in temperature ofthe semiconductor wafers more accurately. A temperature control by thetubular heater 30 and the plane heater 32 can be carried out moreaccurately especially when raising the temperature, so that theprocessing vessel 11 and the semiconductor wafers can be heated to apredetermined temperature more rapidly. Accordingly, it is possible toimprove a throughput by reducing the total time needed for theprocessing.

Due to the provision of the electrically insulative silicon nitridelayer 50A, neither electrically insulative members 62A nor 62B made ofalumina ceramics or the like (see, FIG. 6) is needed for insulating theadjacent metallic wires. Thus, a calorific capacity of the thermocoupleunit 42 is reduced, and thus each thermocouple 42 can more accuratelymeasure transient change in temperature of the semiconductor wafers.

Since all the thermocouples 42 can measure the temperature of thesemiconductor wafers with high accuracy, the zone temperature controlcan be suitably carried out. Thus, the semiconductor wafers can be heattreated with in-plane temperature uniformity and temperature uniformitybetween the semiconductor wafers placed on different levels.

Since the layer 50C, which is in contact with the metallic wires 43A and43B and the connection 44 thereof, is made of inert material thereto, itis possible to prevent the formation of an alloy between the metallicwires 43A and 43B and the connection 44 thereof and the material formingthe film 50, and resultant undesired problems such as breakage of thefilm 50 or the metallic wires 43A and 43B and the connection 44 thereofof each thermocouple 42.

Although embodiments of the present invention have been described above,the present invention is not limited thereto, and various modificationsand changes are possible. For example, the heat treatment apparatusaccording to the present invention is not limited for performing afilm-forming process, and may be configured to perform treatments, suchas oxidation, diffusion and annealing. Further, the heat treatmentapparatus is not limited to a so-called vertical-type one, and may be ofa different type.

EXPERIMENTS

Experiment results performed by employing the heat treatment apparatusshown in FIG. 1 are described below. Thermocouples A to D having belowspecifications were prepared as follows:

[Thermocouple A]

A thermocouple was made by using a platinum wire (43A) and aplatinum-rhodium alloy wire (43B) both having a diameter of 0.3 mm. Afilm (50) composed of a silicon nitride layer (50A), a silicon layer(50B), and a silicon nitride layer (50C) was formed on the surfaces ofthe metallic wires (43A, 43B) and a connection (44) thereof in a mannershown in FIG. 4. A silicon nitride layer (50C) as a first layer(lowermost layer) was 0.2 μm thick, the silicon layer (50B) as a secondlayer was 2 μm thick, and the silicon nitride layer (50A) as a thirdlayer (uppermost layer) was 0.2 μm thick. A reflectivity of thethermocouple (42), at a portion on which the film (50) was formed, for alight of any wavelength within the range of 0.5 to 5 μm was 80% orbelow. This thermocouple is called “thermocouple A”.

[Thermocouple B]

As shown in FIG. 6, a thermocouple unit was made by inserting themetallic wires (43A, 43B) of the thermocouple (A) into tubularelectrically insulative members (62A, 62B) made of alumina ceramics. Thespecifications of the film (50) were the same as those of thethermocouple A. This thermocouple is called “thermocouple B”.

[Thermocouple C]

A comparative thermocouple was made which had the same structure as thatof the thermocouple A but a film was not made on the surfaces of therespective metallic wires and the connection thereof. This thermocoupleis called “thermocouple C”.

[Thermocouple D]

A second comparative thermocouple was made which had the same structureas that of the thermocouple B but a film was not made on the surfaces ofthe respective metallic wires and the connection thereof. Thisthermocouple is called “thermocouple D”.

Each of the four thermocouples A to D was mounted on a center portion ina height direction of the empty wafer boat (17) holding no object to beprocessed thereon. The wafer boat (17) was loaded into the processingvessel (11). Then, the tubular heater (30) and the plane heater (32)were operated for a few minutes with a constant output power. FIG. 5shows the results of the rising of the temperatures of the respectivethermocouples when heated.

As shown in FIG. 5, rising of the temperatures of the thermocouples Aand B according to the present invention were more rapidly than those ofthe comparative thermocouples C and D. Therefore, it was confirmed thata response of a thermocouple can be improved by forming a film having anantireflective function on surfaces of metallic wires and a connectionthereof.

1. A heat treatment apparatus comprising: a processing vessel; a heater adapted to heat an object to be processed placed in the processing vessel; a temperature sensor unit arranged in the processing vessel; and a controller adapted to control an output power of the heater based on a temperature data measured by the temperature sensor unit, wherein the temperature sensor unit includes a protective tube and a thermocouple having a pair of metallic wires contained in the protective tube and connected to each other at a connection, and wherein a film having an antireflective function is formed at least on a surface of the connection of the metallic wires.
 2. The heat treatment apparatus according to claim 1, wherein: the film having the antireflective function is formed of a plurality of layers; and an uppermost layer, with respect to a direction of thickness of the film, of the plurality of layers is electrically insulative.
 3. The heat treatment apparatus according to claim 2, wherein a layer of the plurality of layers directly contacting the metallic wires of the thermocouple is made of a material inert to the metallic wires.
 4. The heat treatment apparatus according to claim 3, wherein the plurality of layers include layers made of silicon nitride as the uppermost layer and the layer directly contacting the metallic wires of the thermocouple, and include a layer made of silicon arranged between the layers made of silicon nitride.
 5. The heat treatment apparatus according to claim 4, wherein the layer made of silicon is doped with phosphate.
 6. The heat treatment apparatus according to claim 1, wherein a reflectivity of a portion, on which is the film is formed, of the thermocouple for a light of any wavelengths within a range of 0.5 to 5 μm is 80% or below.
 7. The heat treatment apparatus according to claim 1, wherein: the object is a silicon wafer; the film includes a plurality of layers; the plurality of layers includes a layer made of silicon; and the layer made of silicon has a thickness greater than any layers of the plurality of layers other than the layer made of silicon.
 8. The heat treatment apparatus according to claim 7, wherein the layer made of silicon is doped with phosphate.
 9. The heat treatment apparatus according to claim 7, wherein the plurality of layers include an uppermost, electrically insulative layer, with respect to a direction of thickness of the film, and a layer directly contacting the metallic wires of the thermocouple and made of a material inert to the metallic wires, and wherein the layer made of silicon is interposed between the electrically insulative layer and the layer made of the material inert to the metallic wires.
 10. The heat treatment apparatus according to claim 1, wherein: the processing is adapted to load an object holder holding a plurality of objects vertically spaced at intervals; the heater is configured so that the heater is capable of controlling temperatures of respective heating zones defined by dividing the processing vessel into a plurality of areas with respect to a vertical direction; the protective tube extends vertically in the processing vessel; there is a plurality of said thermocouples, and the plurality of thermocouples are arranged at positions respectively corresponding to the heating zones.
 11. The heat treatment apparatus according to claim 1, wherein: a plurality of said thermocouples are contained in the protective tube; and films having the antireflective function are provided entirely on portions of the metallic wires of the thermocouples located within the protective tube, and also have a function of electrically isolating the metallic wires of the thermocouples from each other. 